ICOS ligand (ICOSLG) is also known as B7 homolog 2 (B7-H2), B7-related protein 1 (B7RP-1) and CD antigen CD275, which belongs to the immunoglobulin superfamily and BTN/MOG family. ICOSLG contains one Ig-like C2-type (immunoglobulin-like) domain and one Ig-like V-type (immunoglobulin-like) domain. Isoform 1 is widely expressed, while isoform 2 is detected only in lymph nodes, leukocytes and spleen. B7-H2 is ligand for the T-cell-specific cell surface receptor ICOS. B7-H2 acts as a costimulatory signal for T-cell proliferation and cytokine secretion and induces also B-cell proliferation and differentiation into plasma cells. B7-H2 could play an important role in mediating local tissue responses to inflammatory conditions, as well as in modulating the secondary immune response by co-stimulating memory T-cell function.